In case an information signal is to be transmitted through a channel, it is practically impossible for the channel to have completely ideal transmission characteristics (or frequency characteristics). In order to compensate the aforementioned transmission characteristics, therefore, the receiving portion is equipped typically with an equalizer, a distortion compensation circuit or the like having inverse characteristics with respect to the aforementioned transmission characteristics of the channel. However, in case the transmission characteristics of the channel either are unknown or fluctuate with time, it is necessary to detect the transmission characteristics and/or to control the aforementioned equalizer, compensation circuit or the like in accordance with those fluctuations.
In order to detect the transmission characteristics of the channel, it is theoretically possible to perform measurements by applying an impulse to the input end of the channel and by observing the waveform of the impulse at the output end. Nevertheless, the realization of such a detecting arrangement as a practical system is difficult.
On the other hand, it has been proposed that the impulse response to the channel be detected to effect the distortion compensation of the signal at the channel in the manner disclosed in Japanese Patent Laid-Open Publication No. 49-25385, which is entitled "Impulse Response Compensating System". According to this proposal, however, the impulse response is determined by consideration of the data transmitted, with the assumption that the transmission data is sufficiently arbitrary, and by taking a correlation with the signal received. Therefore, when it is intended to accurately determine the impulse response, an integral operation over a long period of time is required, which fails to satisfy the requirement for a normal reference (e.g., 160 ms in the case of 9600 bps). However, in shortening the integration time, the accuracy in the impulse response detection is deteriorated. Moreover, a number of integrating circuits are required to carry out this proposal, so that the circuit construction becomes complicated.